1. Field of the Invention
The present invention relates to a method of controlling an electrode force to be applied to a welding gun for spot welding (hereinafter called a spot welding gun) which is used mainly by mounting it on a robot.
2. Description of the Related Art
As this kind of spot welding gun, there has hitherto been known one as disclosed in Japanese Published Unexamined Patent Application No. 328267/1994. Namely, on a gun main body there are provided a stationary electrode tip which is immovable relative to the gun main body, and a movable electrode tip which is operated by a pressing (or pressurizing) source (i.e., a source for applying an electrode force) to open and close relative to the stationary electrode tip. The gun main body is arranged to be movable relative to a gun support bracket, by means of a driving source, in a direction in which both the electrode tips look toward each other.
In the above-described conventional spot welding gun, the movable electrode tip can be brought into contact with a workpiece by the pressing source, and the stationary electrode tip can be contacted with the workpiece by the movement of the gun main body by means of the driving source, whereby a so-called equalizing function can be obtained.
In the above-described spot welding gun, the electrode force of the pressing source functions or operates equally on the movable electrode tip and on the stationary electrode tip via the gun main body. However, the electrode force by the driving source is added to the stationary electrode tip. In particular, when the relative position of the workpiece relative to the spot welding gun deviates toward the stationary electrode tip due to an error in the setting position of the workpiece or to an error in positioning the robot, the electrode force to be added to the stationary welding gun increases. As a result, there will occur an increase in the difference between the electrode force of the stationary electrode tip to the workpiece and the electrode force of the movable electrode tip to the workpiece.
If a difference occurs between the electrode forces of both the electrode tips, the contact resistance between the electrode tip of the smaller electrode force and the workpiece becomes larger than the contact resistance between the electrode tip of the larger electrode force and the workpiece. Therefore, the center of heat generation of the workpiece during electric charging deviates toward the electrode tip of the smaller electrode force. Consequently, as shown in FIG. 9B, the portion with the largest diameter portion of a nugget N (i.e., that portion of the workpiece which is melted by heat generation through resistance) deviates toward the electrode tip of the smaller electrode force (upper side in the figure). The diameter of the nugget at the welded surfaces of the workpiece W thus becomes smaller, and the shear weld strength of the workpiece W lowers. Further, if the welding electric current is increased so that the diameter of the nugget at the welded surfaces of the workpiece W attains a predetermined value, the upper sheet of the workpiece W melts excessively, with the result that spatters splash around and/or the electrode tip adheres to the workpiece through melting. In this specification, the workpiece W is exemplified to be made up of an upper sheet (referred to as W) and a lower sheet (also referred to as W). Unless otherwise defined, the term "workpiece W" is normally intended to cover the combination of the upper sheet and the lower sheet.
In view of the above-described points, the present invention has an object of providing a method of controlling an electrode force of a spot welding gun in which the electrode force of the stationary electrode tip and the electrode force of the movable electrode tip are equalized so that welding of high quality can be performed without giving rise to spattering and/or adhesion of the electrode tips with the workpiece through melting.